We are talking about a small extremely high risk minority here not The total Thai population.
The official statistics for gay men in Bangkok shows that it is 1 third in other words 33%. That is hard facts not someone just making an estimate out of thin air. There are no official statistics for ladyboys but there is no reason why it should be any lower. And the Red Cross anonymous clinic, you know the guys that does most of the testing on thais in Bangkok and also treat a very large part of everyone living with HIV in Bangkok, both thai and foreign are saying that the percentage for transsexuals are likely even higher than for the gay population.
So we are talking way over 30%. There isn't anyone more qualified to do an estimate than these guys.
And in total I think Thailand is at about 1% so way below sub Saharan Africa as you wanted to compare it with.
That said what do you think the percentage of transsexuals infected in sub Saharan Africa would be? Sure higher than the rest of the population right?
Unless every ladyboy is tested I think it's hard to get an accurate percentage, although I'm sure it's higher then the national average, in regards to a transsexual in sub Saharan African, they would be killed, probably beaten to death for being openly TS or gay.